When was the Psalter closed?
Phil Sumpter has posted Erich Zenger’s thoughts on it closing between 200-150 BCE:
1) The editorially placed Psalter framework (Pss 1-2 and 146-150) reflect the language and theology found in Jesus Sirach (175 BCE).
2) The same goes for Qumran’s wisdom text musar lammebin and the “Book of Mysteries” (Tora wisdom, eschatology, ethnic-cosmological dualism, praise of God).
3) The Qumran Pesher 4QMidr.Eschat(a ) (71-63 BCE) combines the sequence of Ps 1:1 and Ps 2:1f. with other Biblical quotes and applies them eschatologically to the Qumran community. For the order of the Psalter to have had such authority, A. Lange reckons it must have accepted with the grounding of the community in 152 BCE.
4) The LXX translation (Jerusalem, 100 BCE?) affirms the MT ordering and the number of Psalms.
5) The paleographic manuscript Masada Psalms b (2 half of 1st cent. BCE) confirms the order.
6) The differing order in 11QPs [a], could, as you say, be due to liturgical usage. It may even have been made to compete with the MT, which would just confirm the dating.
EDIT: Check out John Anderson’s blog here!